Lyse mouse spleen cells or ES-derived hematopoietic cells in 75 µl elution solution. 2. Set up 20 µl PCR reactions with template DNA (82.5 ng or less), 10 mM Tris-HCl, 50 mM KCl, 2.0 mM MgCl2, 1 µM primers, 200 µM dNTPs, and 1 U Taq DNA polymerase. 3. Run the PCR program: 94°C (2 min), 94°C (1 min), 60°C (1 min), 72°C (1.75 min) for 35 cycles, followed by 72°C (10 min). 4. Electrophorese half of each PCR product on 1% agarose gels, stain with ethidium bromide, and evaluate amounts. 5. Detect DH-JH recombination as amplified fragments (1,033 bp, 716 bp, 333 bp) using primers DHL(5’) and J3(3’). Detect germline alleles as a 1,259 bp fragment using primers Mu0(5’) and J3(3’).